rootlessport: drop Pdeathsig in favor of Kill

there is a race condition where the child process is immediately
killed:

[pid 2576752] arch_prctl(0x3001 /* ARCH_??? */, 0x7ffdf612f170) = -1 EINVAL (Invalid argument)
[pid 2576752] access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
[pid 2576752] --- SIGTERM {si_signo=SIGTERM, si_code=SI_USER, si_pid=2576742, si_uid=0} ---
[pid 2576752] +++ killed by SIGTERM +++

this happens because the parent process here really means the "parent
thread".

Since there is no way of running it on the main thread,
let's skip this functionality altogether and use kill(2).

Signed-off-by: Giuseppe Scrivano <[email protected]>

pkg/rootlessport/rootlessport_linux.go

@@ -149,9 +149,6 @@ func parent() error {
 	cmd.Stdout = &logrusWriter{prefix: "child"}
 	cmd.Stderr = cmd.Stdout
 	cmd.Env = append(os.Environ(), reexecChildEnvOpaque+"="+string(opaqueJSON))
-	cmd.SysProcAttr = &syscall.SysProcAttr{
-		Pdeathsig: syscall.SIGTERM,
-	}
 	childNS, err := ns.GetNS(cfg.NetNSPath)
 	if err != nil {
 		return err
@@ -163,6 +160,12 @@ func parent() error {
 		return err
 	}
 
+	defer func() {
+		if err := syscall.Kill(cmd.Process.Pid, syscall.SIGTERM); err != nil {
+			logrus.WithError(err).Warn("kill child process")
+		}
+	}()
+
 	logrus.Info("waiting for initComplete")
 	// wait for the child to connect to the parent
 outer: